Here's why Apple can't make a solar powered iWatch

Newspapers slavered at the thought of a sun-charged iWatch last week, but is solar science really up to wearables' demands yet?

The rumour mill went wild last week, after the New York Times reported that Apple is investigating wireless charging technology for its much-anticipated smartwatch.

If the report is correct it wouldn't be overly suprising – wireless charging using magnetic resonance induction is well established in the gadget world, and would be a fine way of averting the kind of flack that Samsung got for producing the Galaxy Gear, a smartwatch with a battery that lasts less than a full day.

At least you don't need to worry about reading the 24-hour clock.

Unfortunately, the NYT also mentioned that ex-Apple employees revealed the company experimented with solar panels on watches and iPhones at one stage – including an awesome-sounding attempt at building a transparent photovoltaic cell into a glass screen so as to mystically power a watch without giving it the aesthetics of a 1980s pocket calculator.

What an amazing, Apple-like step forward and innovation that would be. And the world's media fell for it.

Here comes the sun

Certain well known international news sites whose names may or may not rhyme with Male-y Fail confused the NYT's measured reporting that Apple had shelved solar plans (although it did advertise for solar engineers recently) with speculation that the sun-loving silicon Sekonda is ready to wear.

Pebble Smartwatch

Sadly, this is very unlikely. And here's why.

Let's take the most basic smartwatch around at the moment: the Pebble. Thanks to iFixit, we know that it has a 3.7V battery which is rated for 130mAh. Multiplying those two numbers together, it gives us a fully charged capacity of 0.48Wh. The Pebble's low-power ARM Cortex-M3 processor, made by STMicro, consumes 32microWatts per MHz of clockspeed, and its maximum clockspeed is believed to be 80MHz (according to the official spec sheet) – but since that can also clock down to almost nothing most of the time it doesn't tell us much and besides, the battery in the Pebble lasts about five days and also powers an e-ink screen and the Bluetooth transciever.

Since we know that the Pebble lasts for about five days on a single charge (or 120 hours-ish), napkin maths tells us that it thus consumes, on average, somewhere in the region of 0.004W. Which doesn't sound like much, does it? Commercially available, cheap solar cells that are roughly the size of a wristwatch strap can produce 0.08W, which would be more than enough to power the Pebble entirely and keep the battery at full charge. Sure, that's under perfect sunny conditions rather than being buried under a shirt sleeve in an indoor office under artificial light, but at least it would extend the battery life, right?

READ MORE: Apple experimenting with solar charging screen for iWatch

More after the break...

Leaving Pebbles on the shore

The problem, however, is the Pebble is a really bad analogy when it comes to a prospective iWatch – unless Apple really has let its ambitions slide. When you look at something more likely to be a competitor to an iWatch, the figures stop making much sense.

“I don't think you could use the Pebble as a benchmark for power requirements,” says South African technologist, Toby Kurien, “Rather look to the Galaxy Gear for that. If I'm not mistaken, that's around a 315mAh battery, probably running at 3.7V. That equates to around 1.2Wh. So you'd need a panel capable of around 0.4W to be able to charge your watch in about three hours or so.”

A quick reminder – 0.4W is still five times more power than a watch strap sized cell running at maximum efficiency. According to Kurien it's likely that a top end panel would produce twice that amount of power under optimal conditions, but that would still take eight hours of direct sunlight to charge the iWatch.

“It is feasible if the idea is that your watch is constantly charging from sun and/or interior lighting and you're not expecting a full charge,” says Kurien, “though my feeling is that it's still unlikely given practical considerations, and how it affects cost and aesthetics.”

But… But… APPLE!

Just one more thing. We all know how Apple likes to pull something unexpected out of the bag that was previously considered impossible within such a time frame and cost envelope. Like a multi-touch mobile screen, or an implausibly sophisticated thumbprint reader. Surely there's some magic it can do with sun-powered tech?

There's one place that would surely know the answer to this – outside of Apple's secretive R&D section, of course – would be the Centre for Solar Energy Research at Glyndŵr University in Wales. If there's a transparent PV panel that can generate more power per square millimetre than existing technology just around the corner, they might – just might – have got wind of it.

So naturally we asked, and Senior Research Lecturer Dr Vincent Barrioz was happy to look at our numbers. “Never say never, but the figures you are quoting [for battery size] appear to be unlikely for fully charging such batteries,” Barrioz said, “It may be able to top-charge the battery, but considering that the PV panel would be inserted within the size of the smartwatch and transparent (ie. converting only photons from the UV and/or infrared range), I am not aware of that technology being currently available or close to market.

“If it is, I would certainly be interested to know more about it.”

[Apple iWatch concept designs by Martin Hajek and Todd Hamilton]

READ MORE: 5 things we want from the Apple iWatch

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